How To Read A Photoelectron Spectrum

Photoelectron spectroscopy (PES) is a powerful and versatile technique used to probe the electronic structure of matter. By measuring the kinetic energies and intensities of photoelectrons emitted from a sample irradiated with photons of known energy, we can obtain detailed information about the binding energies of electrons in atoms, molecules, and solids. Understanding how to read and interpret a photoelectron spectrum is crucial for researchers in various fields, including chemistry, physics, materials science, and surface science.

Understanding the Basics of Photoelectron Spectroscopy

Before diving into the interpretation of photoelectron spectra, it's important to grasp the fundamental principles behind the technique.

• The Photoelectric Effect: At the heart of PES lies the photoelectric effect, first explained by Albert Einstein. When a photon strikes a material, it can transfer its energy to an electron. If the photon energy is sufficient to overcome the electron's binding energy (EB), the electron will be ejected from the material with a kinetic energy (KE). This relationship is described by the equation:

  • KE = hν - EB - Φ

  where:

  • KE is the kinetic energy of the emitted photoelectron
  • hν is the energy of the incident photon (where h is Planck's constant and ν is the frequency of the photon)
  • EB is the binding energy of the electron
  • Φ is the work function of the material (the minimum energy required to remove an electron from the solid to vacuum)

• The Photoelectron Spectrum: A photoelectron spectrum is a plot of the number of photoelectrons detected as a function of their binding energy. The x-axis represents the binding energy, typically in electron volts (eV), and the y-axis represents the intensity or count rate of the detected photoelectrons. Each peak in the spectrum corresponds to electrons originating from a specific electronic level within the sample.

• Types of Photoelectron Spectroscopy: There are two main types of PES:

  • X-ray Photoelectron Spectroscopy (XPS): Also known as Electron Spectroscopy for Chemical Analysis (ESCA), XPS uses X-ray photons to excite core-level electrons. Core-level electrons are those in the inner shells of an atom and are highly sensitive to the chemical environment. XPS is widely used to determine the elemental composition, chemical states, and electronic structure of materials. Typical X-ray sources used in XPS include Mg Kα (1253.6 eV) and Al Kα (1486.6 eV).
  • Ultraviolet Photoelectron Spectroscopy (UPS): UPS uses ultraviolet (UV) photons to excite valence electrons. Valence electrons are those in the outermost shells of an atom and are responsible for chemical bonding. UPS provides information about the valence band structure, electronic states, and work function of materials. Common UV sources used in UPS include He I (21.22 eV) and He II (40.81 eV).

Step-by-Step Guide to Reading a Photoelectron Spectrum

Now, let's break down the process of reading and interpreting a photoelectron spectrum.

Step 1: Understanding the Spectrum's Axes and Overview

• X-axis (Binding Energy): The x-axis displays the binding energy (EB) in electron volts (eV). Note that the x-axis typically runs from high binding energy on the left to low binding energy on the right. This is because electrons with lower binding energies are easier to remove. Carefully note the energy range displayed, as this dictates what core levels or valence band features are accessible.
• Y-axis (Intensity/Counts): The y-axis represents the intensity or number of photoelectrons detected at each binding energy. Higher intensity indicates a greater number of electrons originating from that specific energy level. This intensity is directly related to the concentration of that element and its photoionization cross-section at the incident photon energy.
• Survey Scan: Begin by examining the survey scan (also sometimes called a "wide scan"). This is a low-resolution scan covering a broad binding energy range (e.g., 0-1400 eV for XPS). The survey scan identifies all the elements present in the sample by the presence of peaks at specific binding energies characteristic of each element. It also allows for a preliminary assessment of the sample's composition and purity.

Step 2: Identifying the Elements Present

• Peak Identification: Each element has a unique set of core-level binding energies. By comparing the observed peak positions to standard reference tables or databases (e.g., the NIST XPS Database), you can identify the elements present in the sample. Keep in mind that the exact binding energy of a core level can shift slightly depending on the chemical environment of the atom (see chemical shift below).
• Core Level Designation: Peaks are labeled according to the core level from which the photoelectrons originate (e.g., C 1s, O 1s, N 1s, Fe 2p). The notation indicates the principal quantum number (n = 1, 2, 3...), the angular momentum quantum number (l = s, p, d, f), and the total angular momentum quantum number (j = l ± 1/2). For example, the Fe 2p peak is split into two components, Fe 2p3/2 and Fe 2p1/2, due to spin-orbit coupling.
• Quantification: The area under each peak is proportional to the concentration of that element in the sample, taking into account factors such as the photoionization cross-section, analyzer transmission function, and mean free path of the photoelectrons. Software packages are commonly used to perform quantitative analysis. It's important to note that XPS is a surface-sensitive technique, so the elemental composition determined by XPS represents the composition of the outermost few nanometers of the sample.

Step 3: Determining the Chemical States

• Chemical Shifts: One of the most powerful aspects of XPS is its ability to provide information about the chemical states of elements. The binding energy of a core-level electron is sensitive to the chemical environment surrounding the atom. This change in binding energy is known as a chemical shift. For example, an oxygen atom in an oxide will have a different O 1s binding energy than an oxygen atom in a hydroxide.
• Analyzing Peak Shapes: The shape of a peak can also provide information about the chemical state. For example, a peak may be broadened or asymmetric due to the presence of multiple chemical states.
• Curve Fitting (Deconvolution): To identify and quantify the different chemical states, it is often necessary to perform curve fitting or peak deconvolution. This involves fitting a series of peaks (usually Gaussian or Lorentzian functions) to the experimental data. The position, width, and area of each peak are adjusted until the overall fit matches the experimental spectrum. This process requires careful consideration of the expected chemical states and can be complex. Software packages provide tools for performing peak fitting, but it's essential to understand the underlying principles and limitations. Constraining the peak fitting parameters based on known chemical information (e.g., fixing the area ratio of spin-orbit split peaks) improves the accuracy of the fit.
• Reference Spectra: Comparing the observed binding energies and peak shapes to reference spectra of known compounds is crucial for accurate chemical state identification. Reference spectra can be found in databases or obtained by measuring standards of well-defined compounds.

Step 4: Identifying and Interpreting Multiplets and Shake-up Satellites

• Multiplet Splitting: In some cases, core-level peaks can be split into multiple components due to multiplet splitting. This phenomenon occurs in elements with unpaired valence electrons and arises from the interaction between the core-hole created during photoemission and the unpaired valence electrons. Multiplet splitting can provide information about the electronic structure and magnetic properties of the material. The splitting patterns are element and oxidation state specific and can be used for fingerprinting.
• Shake-up Satellites: In addition to the main photoelectron peaks, satellite peaks may appear at higher binding energies. These shake-up satellites arise from the simultaneous excitation of a valence electron during the photoemission process. The energy difference between the main peak and the satellite peak corresponds to the excitation energy of the valence electron. Shake-up satellites are common in transition metal compounds and aromatic molecules and can provide information about the electronic structure and bonding characteristics of the material. The intensity and position of shake-up satellites are sensitive to the chemical environment and can be used to distinguish between different chemical states.
• Auger Peaks: Although PES primarily measures photoelectrons, Auger electrons are also emitted during the relaxation process following core-level ionization. Auger peaks can appear in the spectrum and may overlap with photoelectron peaks. Identifying Auger peaks is important for accurate data analysis. Auger peaks are typically broader than photoelectron peaks and their kinetic energy is independent of the incident photon energy.

Step 5: Analyzing Valence Band Spectra (UPS)

• Valence Band Structure: UPS provides information about the electronic structure of the valence band, which is the energy range containing the electrons involved in chemical bonding. The shape and features of the valence band spectrum reflect the density of electronic states (DOS) as a function of energy.

• Work Function Determination: The work function (Φ) is the minimum energy required to remove an electron from the solid to vacuum. In UPS, the work function can be determined from the low-binding energy cutoff of the valence band spectrum and the incident photon energy:

  • Φ = hν - Width

  where Width is the width of the spectrum.

• Identifying Surface States: UPS is sensitive to surface electronic states, which can differ significantly from the bulk electronic states. Surface states can appear as distinct features in the valence band spectrum and can provide information about the surface chemistry and electronic properties of the material.

Step 6: Considering Experimental Artifacts and Limitations

• Charging Effects: Non-conductive samples can develop a surface charge during photoemission, which can shift the entire spectrum to higher binding energies. This effect is known as charging. To correct for charging, a reference peak (e.g., C 1s from adventitious carbon) is often used to calibrate the spectrum. Alternatively, charge neutralization techniques, such as using a low-energy electron flood gun, can be employed.
• Surface Sensitivity: PES is a surface-sensitive technique, with a typical probing depth of a few nanometers. This means that the spectrum primarily reflects the composition and electronic structure of the surface region. Surface contamination, oxidation, or other surface modifications can significantly affect the spectrum.
• Data Processing Artifacts: Improper data processing, such as incorrect background subtraction or peak fitting, can lead to erroneous results. It's crucial to use appropriate data processing techniques and to carefully evaluate the reliability of the results.
• Instrumental Resolution: The resolution of the spectrometer limits the ability to resolve closely spaced peaks. Higher resolution instruments can provide more detailed information about the electronic structure, but they may also require longer acquisition times.

Step 7: Data Interpretation and Conclusion

After analyzing the spectrum, the final step is to interpret the data and draw conclusions about the sample's composition, chemical states, and electronic structure. This involves synthesizing the information obtained from the various steps described above and relating it to the known properties of the material. It's important to consider the limitations of the technique and to support the conclusions with other experimental data or theoretical calculations. A well-supported interpretation should provide a comprehensive understanding of the material's electronic structure and its relationship to its physical and chemical properties.

Examples of Photoelectron Spectra Interpretation

Example 1: Analyzing a TiO2 XPS Spectrum

A typical XPS spectrum of titanium dioxide (TiO2) will show peaks corresponding to Ti 2p and O 1s core levels. The Ti 2p peak is split into two components, Ti 2p3/2 and Ti 2p1/2, due to spin-orbit coupling. The binding energies of these peaks can be used to determine the oxidation state of titanium. For example, in stoichiometric TiO2, the Ti 2p3/2 peak is typically located at around 458.5 eV, indicating that titanium is in the Ti4+ oxidation state. If the sample contains oxygen vacancies or defects, the Ti 2p peak may shift to lower binding energies, indicating the presence of Ti3+ species. The O 1s peak typically appears at around 530.0 eV and corresponds to lattice oxygen. The presence of a shoulder at higher binding energy (around 532 eV) may indicate the presence of hydroxyl groups or adsorbed water on the surface.

Example 2: Analyzing a Gold (Au) UPS Spectrum

A UPS spectrum of gold (Au) will exhibit a characteristic valence band structure with features arising from the Au 5d and 6s electronic states. The Fermi level is clearly visible at 0 eV binding energy. The work function can be determined from the width of the spectrum. The shape and position of the Au 5d bands are sensitive to the surface structure and can be used to study surface reconstructions or adsorption of molecules.

Common Challenges and Troubleshooting

• Overlapping Peaks: Sometimes, peaks from different elements or chemical states may overlap, making it difficult to accurately identify and quantify them. In such cases, higher resolution spectra or spectral deconvolution may be necessary.
• Low Signal-to-Noise Ratio: If the signal intensity is low, it can be difficult to distinguish peaks from the background noise. Increasing the acquisition time or using a more intense photon source can improve the signal-to-noise ratio.
• Sample Contamination: Surface contamination can significantly affect the spectrum. Cleaning the sample surface by sputtering or other methods may be necessary to obtain accurate results. However, sputtering can also alter the chemical composition of the surface, so it's important to use it with caution.
• Instrumental Problems: Malfunctions of the spectrometer, such as detector problems or vacuum leaks, can affect the quality of the data. Regularly calibrating and maintaining the instrument is essential for reliable measurements.

Conclusion

Reading and interpreting photoelectron spectra requires a solid understanding of the underlying principles of the technique, careful data analysis, and a critical evaluation of the results. By following the steps outlined in this guide, researchers can extract valuable information about the composition, chemical states, and electronic structure of materials. While PES can provide powerful insights into the nature of matter, it's important to be aware of its limitations and potential artifacts. Combining PES with other surface-sensitive techniques and theoretical calculations can provide a more complete and accurate picture of the material's properties. Mastering the art of photoelectron spectroscopy is a valuable skill for scientists and engineers working in a wide range of disciplines.